In many older vehicles, and contemporary economy and sports vehicles, no power-assisted steering device is provided. In such vehicles, considerable effort may be expended to rotate the steering wheel. Such vehicles may not have a power assisted steering device for many reasons, such as weight conservation and cost reduction. A common problem with non-power assist steering vehicles is that there is a trade off between low speed turning effort and high speed steering stability. As the number of turns lock-to-lock increases, turning effort decreases.
As is commonly known, steering ratio is generally defined by the ratio of the rotating angle of the steering wheel to the angle by which the dirigible wheels are turned as a result of rotating the steering wheel by the rotating angle. Generally, it is preferable when the vehicle is at rest or moving at a low speed to select the steering ratio so that the dirigible wheels can be turned through a relatively large angle with a small amount of driver-supplied input torque to the steering wheel, thereby reducing the steering effort required of the driver. On the other hand, while the vehicle runs at high speed, the steering ratio should be so selected as to prevent oversteering and to minimize the influence of crosswinds, adverse road contours and the like, thereby providing steering stability.
In conventional steering devices, the steering ratio has a fixed value over almost the entire range of vehicle speeds, thereby making it impossible to meet the requirements of low steering effort at low speeds together with high speed steering stability. The steering ratio is conventionally determined at an intermediate value as a compromise between these two conflicting requirements.
There has been disclosed in the Mazda U.S. Pat. Nos. 4,524,842 and 4,522,279 a variable ratio power steering device in which the steering ratio can be altered according to vehicle speed. However, the inventions disclosed by these patents are adaptable only to vehicles which are provided with a power-assisted steering system. Additionally, the approach taken in these disclosures calls for a system which adds considerably to vehicle weight, requires a number of moving parts, and is relatively expensive to install.
It would therefore be desireable to provide a non-power assisted steering system which can be modified to provide variable effort steering, in which there are no additional moving parts. It would also be useful to provide a variable effort steering system which allows the use of steering gears which are suitable for providing a decrease in the amount of low speed steering effort required of the driver to maneuver the vehicle, while affording high speed steering stability by increasing steering effort. As a result, such a system would readily improve vehicles which do not have power assisted steering equipment.